This invention relates to coatings of the type used to protect components exposed to high temperature environments, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to coatings containing gamma-prime (γ′) phase nickel aluminide, either the gamma (γ-Ni) phase or beta (β) phase nickel aluminide, and a limited but effective amount of a platinum-group metal.
Certain components of the turbine, combustor and augmentor sections susceptible to damage by oxidation and hot corrosion attack are typically protected by an environmental coating and optionally a thermal barrier coating (TBC), in which case the environmental coating is termed a bond coat that in combination with the TBC forms what may be termed a TBC system. Because the maximum design temperature of a component is generally limited by the maximum allowable temperature of its environmental coating or bond coat (in the event of TBC spallation), any improvement in temperature capability of an environmental coating or bond coat results in a higher maximum operating temperature (and/or increased durability) for the component.
Environmental coatings and TBC bond coats are often formed of an oxidation-resistant aluminum-containing alloy or intermetallic whose aluminum content provides for the slow growth of a strong adherent continuous aluminum oxide layer (alumina scale) at elevated temperatures. This thermally grown oxide (TGO) provides protection from oxidation and hot corrosion, and in the case of a bond coat promotes a chemical bond with the TBC. However, a thermal expansion mismatch exists between metallic bond coats, their alumina scale and the overlying ceramic TBC, and peeling stresses generated by this mismatch gradually increase over time to the point where TBC spallation can occur as a result of cracks that form at the interface between the bond coat and alumina scale or the interface between the alumina scale and TBC. More particularly, coating system performance and life have been determined to be dependent on factors that include stresses arising from the growth of the TGO on the bond coat, stresses due to the thermal expansion mismatch between the ceramic TBC and the metallic bond coat, the fracture resistance of the TGO interface (affected by segregation of impurities, roughness, oxide type and others), and time-dependent and time-independent plastic deformation of the bond coat that leads to rumpling of the bond coat/TGO interface. As such, advancements in TBC coating system have been concerned in part with delaying the first instance of oxide spallation, which in turn is influenced by the above strength-related factors.
Environmental coatings and TBC bond coats in wide use include alloys such as MCrAlX overlay coatings (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element), and diffusion coatings that contain aluminum intermetallics, predominantly beta-phase nickel aluminide and platinum aluminides (PtAl). In contrast to the aforementioned MCrAlX overlay coatings, which are metallic solid solutions containing intermetallic phases, the NiAl beta phase is an intermetallic compound present within nickel-aluminum compositions containing about 25 to about 60 atomic percent aluminum. Because TBC life depends not only on the environmental resistance but also the strength of its bond coat, bond coats capable of exhibiting higher strength have been developed, notable examples of which include beta-phase NiAl overlay coatings (as opposed to diffusion coatings) disclosed in commonly-assigned U.S. Pat. No. 5,975,852 to Nagaraj et al., U.S. Pat. No. 6,153,313 to Rigney et al., U.S. Pat. No. 6,255,001 to Darolia, U.S. Pat. No. 6,291,084 to Darolia et al., U.S. Pat. No. 6,620,524 to Pfaendtner et al., and U.S. Pat. No. 6,682,827 to Darolia et al. These intermetallic overlay coatings, which preferably contain a reactive element (such as zirconium and/or hafnium) and/or other alloying constituents (such as chromium), have been shown to improve the adhesion and spallation resistance of a ceramic TBC. The presence of reactive elements such as zirconium and hafnium in beta-phase NiAl overlay coatings has been shown to improve environmental resistance as well as strengthen the coating, primarily by solid solution strengthening of the beta-phase NiAl matrix. However, if the solubility limits of the reactive elements are exceeded, precipitates of a Heusler phase (Ni2AlZr (Hf, Ti, Ta)) can form that can drastically lower the oxidation resistance of the coating due to preferential internal oxidation of these precipitates.
The suitability of environmental coatings and TBC bond coats formed of NiAlPt to contain both gamma phase (γ-Ni) and gamma-prime phase (γ′-Ni3Al) is reported in U.S. Patent Application Publication No. 2004/0229075 to Gleeson et al. The NiAlPt compositions evaluated by Gleeson et al. contained less than about 23 atomic percent (about 9 weight percent or less) aluminum, and between about 10 and 30 atomic percent (about 28 to 63 weight percent) platinum. Additions of reactive elements are also contemplated by Gleeson et al. According to Gleeson et al., the compositions were predominantly made up of the gamma and gamma prime phases, with substantially no beta phase. NiAlPt compositions have been shown to be substantially free of the rumpling phenomenon associated with TBC coating failure on PtAl bond coats, and the high levels of platinum in these coatings can result in excellent oxidation performance. Furthermore, the relatively low aluminum content of these NiAlPt compositions reduces and potentially eliminates the formation of topologically close-packed (TCP) phases, which form a particularly detrimental type of diffusion zone known as a secondary reaction zone (SRZ) observed in newer generation high strength superalloys when protected by high aluminum-activity coatings.
Even with the above advancements, there remains a considerable and continuous effort to further increase the service life of environmental coatings and TBC systems.